Pathogenetic Classification

Type A side effects: predictable and dose dependent

Definition: Exaggerated but otherwise normal pharmacological action of a drug given in the usual therapeutic doses. Side effects are predictable, usually dose dependent. Their prevalence and morbidity are high and the mortality usually low.


Possible causes:

  • Pharmaceutical causes
    • Drug quality
    • Drug release
  • Pharmacokinetic causes
    • Drug absorption
    • Drug distribution
    • Drug elimination
    • Drug receptors
    • Homoeostatic mechanisms
    • Neoplastic and teratological reasons



Type B side effects: unpredictable and not dose dependent

Definition: Totally aberrant that are not to be expected from the pharmacological actions of a drug when given in the usual therapeutic doses to a patient whose body handles the drug in the normal way. Type B reactions are often serious.


Possible causes:

  • Pharmaceutical causes
  • Pharmacokinetic causes
  • Pharmacodynamic causes
    • Genetic causes for abnormal response
    • Immunological reasons for abnormal response



Type U: mechanism unknown


See A. Davies: Textbook on Adverse Effects of Drugs